Methods for Forming Contact Plugs with Reduced Corrosion

ABSTRACT

A method includes forming an ILD to cover a gate stack of a transistor. The ILD and the gate stack are parts of a wafer. The ILD is etched to form a contact opening, and a source/drain region of the transistor or a gate electrode in the gate stack is exposed through the contact opening. A conductive capping layer is formed to extend into the contact opening. A metal-containing material is plated on the conductive capping layer in a plating solution using electrochemical plating. The metal-containing material has a portion filling the contact opening. The plating solution has a sulfur content lower than about 100 ppm. A planarization is performed on the wafer to remove excess portions of the metal-containing material. A remaining portion of the metal-containing material and a remaining portion of the conductive capping layer in combination form a contact plug.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.16/213,326, entitled “Methods for Forming Contact Plugs with ReducedCorrosion,” filed on Dec. 7, 2018, which is a continuation of U.S.patent application Ser. No. 15/492,113, entitled “Methods for FormingContact Plugs with Reduced Corrosion,” filed on Apr. 20, 2017, now U.S.Pat. No. 10,186,456 issued Jan. 22, 2019, which applications areincorporated herein by reference.

BACKGROUND

In the manufacturing of integrated circuits, contact plugs are used forconnecting to the source and drain regions and the gates of transistors.The source/drain contact plugs were typically connected to source/drainsilicide regions, whose formation includes forming contact openings toexpose source/drain regions, depositing a metal layer, performing ananneal to react the metal layer with the source/drain regions, fillingtungsten into the remaining contact opening, and performing a ChemicalMechanical Polish (CMP) to remove excess tungsten. A cleaning is thenperformed. In the CMP and the subsequent cleaning processes, the topsurface of the contact plug may suffer from dishing and corrosion.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 12 are cross-sectional views of intermediate stages inthe formation of a transistor and contact plugs in accordance with someembodiments.

FIG. 13 illustrates a cross-sectional view of a transistor and contactplugs in accordance with some embodiments.

FIG. 14 illustrates a process flow for forming a transistor inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A transistor having contact plugs electrically connected to asource/drain region and a gate electrode, and the method of forming thesame are provided in accordance with various exemplary embodiments. Theintermediate stages of forming the transistor are illustrated. Thevariations of some embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

FIGS. 1 through 12 are cross-sectional views of intermediate stages inthe formation of a transistor and the respective contact plugs inaccordance with some exemplary embodiments. The steps shown in FIGS. 1through 12 are also illustrated schematically in the process flow 200 asin FIG. 14. Referring to FIG. 1, the initial structures on wafer 10 areformed. Wafer 10 includes substrate 20, which may be formed of asemiconductor material such as silicon, silicon germanium, siliconcarbon, a III-V compound semiconductor material, or the like. Substrate20 may be a bulk substrate or a Semiconductor-On-Insulator (SOI)substrate.

Gate stacks 26A and 26B, which are collectively referred to as gatestacks 26, are formed over substrate 20. In accordance with someembodiments of the present disclosure, gate stacks 26A and 26B areformed as gate stack strips (in a top view of wafer 10) havinglengthwise directions parallel to each other. Each of gate stacks 26Aand 26B may include gate dielectric 24, gate electrode 28 over gatedielectric 24, and hard mask 38 over gate electrode 28. In accordancewith some embodiments of the present disclosure, gate stacks 26 arereplacement gate stacks, which are formed by forming dummy gate stacks(not shown), removing the dummy gate stacks to form recesses, andforming the replacement gates in the recesses. As a result, each of gatedielectrics 24 includes a bottom portion underlying the respective gateelectrode 28, and sidewall portions on the sidewalls of the respectivegate electrode 28. The sidewall portions form rings encircling therespective gate electrodes 28.

In accordance with some embodiments of the present disclosure, sourceand drain regions 22 (referred to as source/drain regions 22hereinafter) are formed to extend into substrate 20, and is formedbefore the formation of Contact Etch Stop Layer (CESL) 34, Inter-LayerDielectric (ILD) 36, and the contact opening therein. In accordance withalternative embodiments, source/drain regions 22 are formed after theformation of the contact opening as shown in FIG. 2. One or more ofsource/drain regions 22 may be a common source region or a common drainregion shared by neighboring gate stacks including 26A and 26B.Accordingly, gate stack 26A may form a first transistor in combinationwith the source/drain regions 22 on the opposite sides of gate stack26A, and gate stack 26B may form a second transistor in combination withthe source/drain regions 22 on the opposite sides of gate stack 26B. Thefirst transistor and the second transistor may be electrically connectedin parallel to act as a single transistor.

Gate dielectric 24 may be a single layer or a composite layer thatincludes a plurality of layers. For example, gate dielectric 24 mayinclude an interfacial oxide layer and a high-k dielectric layer overthe oxide layer. The oxide layer may be a silicon oxide layer formedthrough thermal oxidation or chemical oxidation. The high-k dielectriclayer may have a k value greater than 7, or even greater than 20.Exemplary high-k dielectric materials include hafnium oxide, zirconiumoxide, lanthanum oxide, and the like.

In accordance with some embodiments of the present disclosure, each gateelectrode 28 has a single-layer structure formed of a homogeneousconductive material. In accordance with alternative embodiments, eachgate electrode 28 has a composite structure including a plurality oflayers formed of TiN, TaSiN, WN, TiAl, TiAlN, TaC, TaN, aluminum, oralloys thereof. The formation of gate electrodes 28 may include PhysicalVapor Deposition (PVD), Metal-Organic Chemical Vapor Deposition (MOCVD),and/or other applicable methods. Hard masks 38 may be formed of siliconnitride or silicon oxynitride, for example.

In accordance with alternative embodiments of the present disclosure,rather than being replacement gate stacks, gate stacks 26A and 26B areformed by depositing a blanket gate dielectric layer and a blanket gateelectrode layer (such as a polysilicon layer), and then patterning theblanket gate dielectric layer and the blanket gate electrode layer.

Referring again to FIG. 1, Contact Etch Stop Layer (CESL) 34 is formedto cover substrate 20, and may extend on the sidewalls of gate spacers30. In accordance with some embodiments of the present disclosure, CESL34 is formed of silicon nitride, silicon carbide, or other dielectricmaterials. Inter-Layer Dielectric (ILD) 36 (alternatively referred to asILD0 36) is formed over CESL and gate stacks 26A and 26B. ILD 36 may beformed of an oxide such as Phospho-Silicate Glass (PSG), Boro-SilicateGlass (BSG), Boron-Doped Phospho-Silicate Glass (BPSG), Tetra EthylOrtho Silicate (TEOS) oxide, or the like. The formation may include, forexample, Chemical Vapor Deposition (CVD), Flowable CVD (FCVD), spin-oncoating, or the like. ILD 36 may include a first layer having a topsurface level with the top surfaces of gate stacks 26A and 26B, and gatestacks 26A and 26B are replacement gates formed in the first layer. ILD36 may further include a second layer formed over the first layer, andthe second layer is formed after the formation of gate stacks 26A and26B. The first and the second layers may be formed of a same material ordifferent materials, and may or may not have a distinguishable interfacein between.

Referring to FIG. 2, ILD 36 and CESL 34 are etched to form source/draincontact opening 40. The respective step is illustrated as step 202 inthe process flow shown in FIG. 14. Source/drain region 22 (if alreadyformed) is exposed to contact opening 40. The etching is anisotropic, sothat the sidewalls of opening 40 are substantially vertical.

In accordance with some embodiments in which source/drain regions 22have not been formed yet at this time, a Pre-Amorphization Implantation(PAI) and a source/drain implantation may be performed to formsource/drain regions 22, and the species of the PAI and the implantedimpurity for forming source/drain regions 22 are implanted intosubstrate 20 through opening 40. The PAI may be performed usinggermanium, silicon, or the like, which destroys the lattice structure ofthe implanted regions in order to control the depth of the subsequentsource/drain implantation. The source/drain implantation may beperformed using boron or indium if the respective transistor is a p-typetransistor, or using phosphorous, arsenic, or antimony if the respectivetransistor is an n-type transistor.

In accordance with some embodiments, ILD 36 is formed of a homogenousdielectric material. In accordance with alternative embodiments, asshown in FIG. 2, dashed line 35 is drawn to show that ILD 36 may includelayer 36A and layer 36B over layer 36A. Layers 36A and 36B are bothdielectric layers. Layer 36B may be formed of a dielectric materialdifferent from the material of layer 36A. For example, both layer 36Aand layer 36B may be formed of dielectric materials selected from thesame group of candidate dielectric materials for forming ILD such asPSG, BSG, BPSG, and TEOS, while different materials are selected. Insubsequent formation, layer 36B may be removed, and hence is used as asacrificial layer.

FIG. 3 illustrates the formation of dielectric contact (plug) spacers 44in accordance with some embodiments of the present disclosure. Inaccordance with alternative embodiments, contact spacers 44 are notformed. The formation of contact spacers 44 may include depositing oneor a plurality of conformal dielectric layer(s). The dielectric layerextend into contact opening 40, and includes vertical portions on thesidewalls of ILD 36, and horizontal portions at the bottom of opening 40as well as over ILD 36. The deposition process is performed using aconformal deposition process such as Atomic Layer Deposition (ALD), CVD,or the like, so that the horizontal portions and vertical portions ofthe deposited layer have similar thicknesses. An anisotropic etching isthen performed to remove the horizontal portions of the dielectriclayer, leaving the vertical portions as contact spacers 44. Theanisotropic etching may be performed using ammonia (NH₃) and NF₃ asetching gases. It is noted that contact spacers 44 in the same opening40, when viewed in a top view of wafer 10, are portions of an integratedspacer ring.

In accordance with some embodiments of the present disclosure, spacers44 are formed of a dielectric material that has a high etchingselectivity relative to oxide, so that in subsequent cleaning processes(in which oxides are removed), spacers 44 are not damaged. For example,contact spacers 44 may be formed of silicon nitride, siliconoxy-carbide, silicon oxynitride, or the like.

Referring to FIG. 4, a lithography mask such as photo resist 43 isformed over ILD 36. Photo resist 43 fills source/drain contact opening40 (FIG. 3). Photo resist 43 is then patterned. An etching process(es)is performed using photo resist 43 as an etching mask to etch ILD 36, sothat gate contact openings 41 are formed. The respective step isillustrated as step 204 in the process flow shown in FIG. 14. Hard masks38 are then etched, and openings 41 extend into the space betweenopposite gate spacers 30. Gate electrodes 28 (and possibly gatedielectrics 24) are thus exposed to gate contact openings 40. Inaccordance with some embodiments of the present disclosure, theformation of openings 41 includes an anisotropic etch to etch-throughILD 36, and an isotropic etch (dry etch) or an anisotropic etch (dryetch or wet etch) to etch hard masks 38. The sidewalls of gate spacers30 may be or may not be exposed to openings 42 after the etching.

In accordance with some embodiments, as shown in FIG. 5, contact spacers45 are formed in openings 41. In accordance with alternativeembodiments, contact spacers 45 are omitted. Contact spacers 45 may beformed of a material selected from the same group of candidate materialsfor forming contact spacers 44. Contact spacers 44 and 45 may be formedof a same material or different materials. In accordance with someembodiments, contact spacers 44 and 45 are formed in differentprocesses, each following the formation of the respective contactopenings 40 and 41. In accordance with alternative embodiments, contactspacers 44 and 45 are formed after both contact openings 40 and 41 areformed, and are formed in a common formation process, which includesdepositing a blanket dielectric layer, and then performing ananisotropic etching on the blanket dielectric layer. The formation ofeither one or both of contact spacers 44 and 45 may also be omitted, andthe subsequently formed contact plug(s) will be in contact with ILD 36.

Next, referring to FIG. 6, metal layer 46 is deposited. The respectivestep is illustrated as step 206 in the process flow shown in FIG. 14. Inaccordance with some embodiments of the present disclosure, metal layer46 is a titanium (Ti) layer, which may be formed using Physical VaporDeposition (PVD). Metal layer 46 includes a bottom portion at the bottomof opening 40, and sidewall portions on the sidewall surfaces of ILD 36.Metal layer 46 has two functions. The first function is that the bottomportion of metal layer 46 reacts with the underlying source/drain region22 to form a source/drain silicide region. The second function is thatmetal layer 46 acts as an adhesion layer for the subsequently formedcapping layer.

In accordance with alternative embodiments, openings 40 and 41 arefilled in different processes, and metal layer 46 is filled into opening40, and not into openings 41. Conductive capping layer 48 and metallicmaterial 54, however, are still filled into both openings 40 and 41 inaccordance with these embodiments.

Referring to FIG. 7, conductive capping layer 48 is deposited. Therespective step is illustrated as step 208 in the process flow shown inFIG. 14. Capping layer 48 also acts as a diffusion barrier layer. Inaccordance with some embodiments of the present disclosure, cappinglayer 48 is formed of a metal nitride such as titanium nitride. Cappinglayer 48 may be formed using PVD, CVD, or the like.

FIG. 8 illustrates a silicidation process for forming source/drainsilicide region 50. In accordance with some embodiments of the presentdisclosure, the silicidation process is performed through an anneal,which is represented by arrows 52. The respective step is illustrated asstep 210 in the process flow shown in FIG. 14. The anneal may beperformed through Rapid Thermal Anneal (RTA), furnace anneal, or thelike. Accordingly, the bottom portion of metal layer 46 reacts withsource/drain region 22 to form silicide region 50. The sidewall portionsof metal layer 46 remain after the silicidation process. In accordancewith some embodiments of the present disclosure, the bottom portion ofmetal layer 46 is fully reacted, and the top surface of silicide region50 is in contact with the bottom surface of capping layer 48.

Next, metallic material 54 is filled into the remaining contact openings40 and 41, and the resulting wafer 10 is shown in FIG. 9. The respectivestep is illustrated as step 212 in the process flow shown in FIG. 14.Metallic material 54 may be formed of a cobalt-containing material or atungsten-containing material, which may be formed of pure orsubstantially pure tungsten or cobalt (for example, with an atomicpercentage greater than about 95 percent).

In accordance with some embodiments of the present disclosure, theformation of metallic material 54 is performed through ElectroChemicalPlating (ECP). During an ECP, a plating solution (schematicallyillustrated as 55) is in contact with wafer 10, and a current isconducted through plating solution 55. For example, the plating may beperformed by submerging wafer 10 into the plating solution 55. Inaccordance with some embodiments, plating solution 55 includes ametal-containing chemical such as Boric acid, CoSO₄ in H₂SO₄ andadditional chemical(s) such as organic compounds with C—H and/or N—Hbonds.

Plating solution 55 may include sulfur (S) in its electrolyte. As aresult, the plated metallic material 54 also includes sulfur. The sulfurin metallic material 54 will cause the corrosion of metallic material 54in subsequent steps, as will be discussed in subsequent paragraphs.Accordingly, the sulfur content in the electrolyte is reduced oreliminated before the plating. In accordance with some embodiments ofthe present disclosure, plating solution 55 is free from anysulfur-containing chemical, so that no sulfur will be deposited intometallic material 54. In accordance with alternative embodiments,plating solution 55 is adjusted, so that although there issulfur-containing chemical (such as organic compounds including sulfurand having C—H and/or N—H bonds, the amount of sulfur in platingsolution 55 is lower than 100 Parts Per Million (ppm). Plating solution55 may also be substantially free from sulfur, for example, with theamount of sulfur in plating solution 55 being lower than about 20 ppm orlower than about 10 ppm, so that the corrosion of metallic material 54,if any, will not affect the quality of the resulting contact plugs. Ifplating solution 55 has already been purchased (or provided) and has asulfur content higher than about 100 ppm, plating solution 55 isprocessed to remove sulfur in order to reduce the sulfur content tolower than 100 ppm, and to a desirable level such as lower than about 20ppm or 10 ppm before used for plating, and the plating solution 55 usedfor plating may be free or substantially free from sulfur. Also, platingsolution 55 may have a small amount of sulfur content, which may be morethan about 1 ppm, for example, and hence the sulfur content may be inthe range between about 1 ppm and about 100 ppm, in the range betweenabout 1 ppm and about 20 ppm, or in the range between about 1 ppm andabout 10 ppm. The resulting metallic material 54 may include a traceamount of sulfur, with the amount significantly reduced or fullyeliminated due to the reduction or the elimination of sulfur in platingsolution 55.

The ECP of metallic material 54 may be bottom-up, which means at thebottoms of contact openings 40 and 41 (FIG. 8), the deposition rate ismuch higher than in upper regions such as on the portions of cappinglayer 48 over ILD 36. Accordingly, metallic material 54 fills openings40 and 41, and continues to grow up.

After metallic material 54 is deposited, an anneal is performed, inaccordance with some embodiments of the present disclosure, the annealis performed using Rapid Thermal Anneal (RTA), and the duration of theanneal may be in the range between about 2 minutes and about 10 minutes.The temperature of the anneal may be in the range between about 300° C.and about 500° C. If sulfur exists in metallic material 54, the annealwill cause the diffusion of sulfur, and the sulfur content at theinterface between metallic material 54 and capping layer 48 increases asa result of the diffusion.

Next, a planarization process such as a Chemical Mechanical Polish (CMP)is performed to remove the excess portions of metallic material 54,capping layer 48, and metal layer 46 over ILD 36. Source/drain contactplug 56A and gate contact plugs 56B are thus formed, as shown in FIG.10A. The respective step is illustrated as step 214 in the process flowshown in FIG. 14. FIG. 10A schematically illustrates polish pad 57. Inthe actually CMP process, polish pad 57 may have a size/diameter greaterthan the size of wafer 10. During the CMP process, polish pad 57 mayface up, while wafer 10 may face down and is pressed against polish pad57. Wafer 10 is rotated during the CMP. A slurry (not shown) isdispensed on polish pad 57 during the CMP.

In accordance with some embodiments in which ILD 36 includes layers 36Aand 36B (FIG. 9), the planarization process may be performed until layer36B is fully removed. Accordingly, layer 36B acts as a sacrificial layerprotecting the underlying ILD 36A.

It is found that if sulfur exists in metallic material 54, during theCMP, contact plugs 56A and 56B may be corroded to form recesses 60, asshown in FIG. 10B. Conversely, if no sulfur exists in metallic material54, during the CMP, no corrosion occurs, and the top surfaces of contactplugs 56A and 56B may be planar with no recess formed. The corrosion maybe caused by the reaction of sulfur with the slurry to form sulfuricacid, which corrodes contact plugs 56A and 56B. The anneal furthercauses the concentration of sulfur in the regions close to the interfacebetween metallic material 54 and capping layer 48. Accordingly, theconcentrated sulfur at the interface causes recesses 60 to be deeperadjacent to the interface than in other portions of contact plugs 56Aand 56B, and the recessing profile as shown in FIG. 10B is formed. Theedge portions of recesses 60 have depth D1, and the center portion ofrecess 60 has depth D2 smaller than D1. In accordance with someembodiments, ratio D1/D2 is greater than 2.0. It is appreciated thatdepths D1 and D2 and ratio D1/D2 are related to various factors such asthe sulfur content in metallic material 54, the anneal condition, andthe slurry. Sulfur may also be diffused into a shallow portion ofcapping layer 48. Accordingly, recesses 60 may also expand into metalcapping layer 48.

When no corrosion occurs, or the corrosion is very light, dielectriclayers and conductive features may be formed directly on the wafer 10shown in FIG. 10A. If the corrosion still occurs and is not negligible,metal caps 62 may be formed to fill the recesses 60 as shown in FIG.10B, and the resulting wafer 10 is shown in FIG. 11A. Metal caps 62 thusextend into recesses 60. In accordance with some exemplary embodiments,the top surfaces of metal caps 62 are substantially coplanar with thetop surfaces of ILD 36. Metal caps 62 may be deposited using CVD, and aprecursor is selected so that the formation is selective, and metal caps62 are formed on contact plugs 56A and 56B, and not on ILD 36. Metalcaps 62 may be formed of cobalt, tungsten, nickel, or alloys thereof.Furthermore, metal caps 62 and metallic material 54 may be formed of asame material or different materials. In FIG. 11A and the subsequentfigures, metal caps 62 are shown using dashed lines to indicate they maynot be formed in response to the structure shown in FIG. 10A, or may beformed in response to the structure shown in FIG. 10B.

FIG. 11B illustrates wafer 10 in accordance with some embodiments, andthe CMP is performed until all of ILD 36 over gate spacers 30 areremoved, and gate spacers 30 are exposed. As a result, each of gatecontact plugs 56B is fully in the space between opposite gate spacers30. In accordance with these embodiments, the corrosion of contact plugs56A and 56 may or may not occur, and as the results, metal caps 62 mayor may not be formed, as indicated by the dashed lines for showing metalcaps 62.

In the steps as shown in FIGS. 1 through 11A/11B, transistor 300 isformed. Referring to FIG. 12, etch stop layer 68 is formed, followed bythe formation of dielectric layer 70. In accordance with someembodiments, dielectric layer 70 is an inter-layer dielectric, and henceis alternately referred to as ILD1 70. Etch stop layer 68 may also beomitted in accordance with some embodiments. Accordingly, etch stoplayer 68 is illustrated using dashed lines to indicate it may or may notbe formed. Etch stop layer 68 may be formed of silicon carbide, siliconoxynitride, silicon carbo-nitride, combinations thereof, or compositelayers thereof. Etch stop layer 68 may be formed using a depositionmethod such as CVD, Plasma Enhanced Chemical Vapor Deposition (PECVD),ALD, or the like. ILD1 70 may include a material selected from PSG, BSG,BPSG, Fluorine-doped Silicon Glass (FSG), or TEOS oxide. ILD1 70 mayalso be formed of a non-porous low-k dielectric material, which may be acarbon-containing dielectric material. ILD1 70 may be formed using spincoating, FCVD, or the like, or may be formed using a deposition methodsuch as CVD, PECVD, Low Pressure Chemical Vapor Deposition (LPCVD), orthe like.

FIG. 12 further illustrates the formation of conductive features 72,which may be metal lines, metal vias, metal pads, etc. The formation oflayers 68 and 70 and conductive features 72 is illustrated as step 216in the process flow shown in FIG. 14. In accordance with someembodiments of the present disclosure, conductive feature 72 is acontact plug, and the etch stop layer 68 as shown in FIG. 10 is notformed. In accordance with alternative embodiments, conductive feature72 is a copper via or a copper line, and etch stop layer 68 is formed inaccordance with these embodiments.

The formation of conductive feature 72 may include forming an opening indielectric layers 68 and 70 to expose contact plug 56, filling aconductive material(s) in the opening, and performing a planarization.Conductive features 72 may include conductive adhesion/barrier layers74, and metallic material 76 over adhesion/barrier layers 74.Adhesion/barrier layer 74 may be formed of a material selected fromtitanium, titanium nitride, tantalum, tantalum nitride, combinationsthereof, or multi-layers thereof. Metallic material 76 may be formed oftungsten, copper, aluminum, or alloys thereof, and may be formed usingPVD, Metal-Organic Chemical Vapor Deposition (MOCVD) or plating. Inaccordance with some embodiments, metallic material 76 is formed usingECP, and the respective plating solution may have a sulfur contentsimilar to plating solution 55 (FIG. 9), which has a sulfur contentlower than about 100 ppm, or may be free from sulfur in order to reducecorrosion.

The embodiments of the present disclosure have some advantageousfeatures. By lowering or removing sulfur in the plating solution forforming contact plugs, the corrosion of the contact plugs during CMP isreduced or eliminated. In addition, metal caps may be formed selectivelyto fill the recesses, if any, formed due to the corrosion. The gatecontact plugs may also be formed fully between gate spacers to reducethe electrical short or leakage caused by the misalignment of the metalvias/plugs over the gate contact plugs.

In accordance with some embodiments of the present disclosure, a methodincludes forming an ILD to cover a gate stack of a transistor. The ILDand the gate stack are parts of a wafer. The ILD is etched to form acontact opening, and a source/drain region of the transistor or a gateelectrode in the gate stack is exposed through the contact opening. Aconductive capping layer is formed to extend into the contact opening. Ametal-containing material is plated on the conductive capping layer in aplating solution using electrochemical plating. The metal-containingmaterial has a portion filling the contact opening. The plating solutionhas a sulfur content lower than about 100 ppm. A planarization isperformed on the wafer to remove excess portions of the metal-containingmaterial. A remaining portion of the metal-containing material and aremaining portion of the conductive capping layer in combination form acontact plug.

In accordance with some embodiments of the present disclosure, a methodincludes forming an ILD, and etching the ILD to form a first contactopening and a second contact opening. A source/drain region and a gateelectrode of a transistor are exposed through the first contact openingand the second contact opening, respectively. The method furtherincludes depositing a metal layer extending into both the first contactopening and the second contact opening, and depositing a conductivecapping layer. The conductive capping layer extends into both the firstcontact opening and the second contact opening. A metal-containingmaterial is plated on the conductive capping layer in a plating solutionusing electrochemical plating. The plating solution is substantiallyfree from sulfur. A planarization is performed on the wafer to removeexcess portions of the metal-containing material. Remaining portions ofthe metal-containing material and remaining portions of the conductivecapping layer form a source/drain contact plug and a gate contact plug.

In accordance with some embodiments of the present disclosure, a methodincludes forming an ILD, and etching the ILD to form a contact opening.A source/drain region or a gate electrode of a transistor is exposedthrough the contact opening. The method further includes depositing ametal layer extending into the contact opening, depositing a conductivecapping layer having a first portion extending into the contact opening,and a second portion overlying the ILD, and plating a metal-containingmaterial on the conductive capping layer in a plating solution usingelectrochemical plating. The plating solution is substantially free fromsulfur. A planarization is performed on the wafer to remove excessportions of the metal-containing material. Remaining portions of themetal-containing material and a remaining portion of the conductivecapping layer in combination form a contact plug. A top surface of thecontact plug recesses from an adjacent top surface of the ILD to form arecess due to the planarization. A metal cap is selectively deposited inthe recess.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An integrated circuit structure comprising: asemiconductor region; a gate stack over the semiconductor region; asource/drain region extending into the semiconductor region; asource/drain silicide region over the source/drain region; a contactplug comprising: a titanium-based capping layer comprising: a bottomportion over and electrically connecting to one of the source/drainsilicide region and the gate stack; and sidewall portions over, andconnected to, the bottom portion; and a metal region between thesidewall portions of the titanium-based capping layer; and an additionalcapping layer comprising a first portion overlapping the metal region,and second portions extending into the sidewall portions of thetitanium-based capping layer.
 2. The integrated circuit structure ofclaim 1, wherein the second portions of the additional capping layer areon opposing sides of, and extend lower than, the first portion of theadditional capping layer.
 3. The integrated circuit structure of claim1, wherein the additional capping layer comprises opposite edgescontacting edges of the sidewall portions of the titanium-based cappinglayer.
 4. The integrated circuit structure of claim 1, wherein theadditional capping layer comprises a cobalt-based material.
 5. Theintegrated circuit structure of claim 1, wherein the additional cappinglayer comprises a tungsten-based material.
 6. The integrated circuitstructure of claim 1, wherein the bottom portion of the contact plugcontacts the source/drain silicide region.
 7. The integrated circuitstructure of claim 1, wherein the bottom portion of the contact plugcontacts the gate stack.
 8. The integrated circuit structure of claim 1,wherein the titanium-based capping layer comprises titanium nitride. 9.The integrated circuit structure of claim 1 further comprising adielectric contact spacer encircling the contact plug.
 10. An integratedcircuit structure comprising: a semiconductor region; a gate stack overthe semiconductor region; an Inter-layer Dielectric (ILD) havingportions on opposite sides of the gate stack; and a contact plug in theILD, the contact plug comprising: a metal layer comprising a firstbottom portion, and first sidewall portions over and connected to thefirst bottom portion; a metal nitride layer comprising: a second bottomportion over and contacting the first bottom portion; and secondsidewall portions over, and connected to, the second bottom portion; afirst metal region over and contacting the second bottom portion of themetal nitride layer; and a capping layer overlapping the first metalregion, wherein the capping layer is between the second sidewallportions of the metal nitride layer.
 11. The integrated circuitstructure of claim 10, wherein the contact plug is a gate contact plug,and the first bottom portion is over and contacting the gate stack. 12.The integrated circuit structure of claim 10 further comprising anadditional contact plug over and contacting both of the capping layerand the second sidewall portions of the metal nitride layer.
 13. Theintegrated circuit structure of claim 10, wherein the capping layercomprises: a first portion having a bottom surface contacting a topsurface of the first metal region; and second portions on opposing sidesof the first portion and extending lower than the first portion.
 14. Theintegrated circuit structure of claim 13, wherein the second portionsextend laterally into the second sidewall portions of the metal nitridelayer.
 15. The integrated circuit structure of claim 10 furthercomprising a dielectric contact spacer encircling the contact plug. 16.The integrated circuit structure of claim 15 further comprising: gatespacers on opposite sidewall of the gate stack; and a dielectric hardmask between the gate spacers and overlapping the gate stack, whereinthe dielectric contact spacer and the contact plug penetrate through thedielectric hard mask to contact the gate stack.
 17. An integratedcircuit structure comprising: a contact plug comprising: a metal region;and a metal nitride layer comprising first sidewall portions on opposingsidewalls of the metal region; a metal layer comprising second sidewallportions on opposing sides of a combine region, wherein the combinedregion comprises the metal region and the metal nitride layer; and acapping layer overlapping the metal region and the first sidewallportions of the metal nitride layer, wherein a first top surface of thefirst sidewall portions of the metal nitride layer is at a same level asa second top surface of the capping layer.
 18. The integrated circuitstructure of claim 17 further comprising an additional layer over andcontacting both of the capping layer and the first sidewall portions ofthe metal nitride layer, wherein the additional layer comprises a metalnitride.
 19. The integrated circuit structure of claim 17, wherein themetal region forms a vertical interface with the metal nitride layer,and the capping layer overlaps the vertical interface.
 20. Theintegrated circuit structure of claim 17, wherein the capping layercomprises a first portion, and second portions on opposing sides of thefirst portion, and the second portions extend lower than the firstportion.